This invention relates to apparatus for and a method of providing a controlled concentration of a gas in a flow of liquid. A typical example of an application of such apparatus is in the calibration of monitors used for determining the concentration of gases dissolved in water. Apparatus according to the invention may be used to provide a known, controlled concentration of the gas, to afford a fixed point calibration of the monitor. An advantageous use of apparatus according to the invention lies in the provision of a controlled concentration of oxygen in a flow of water for use in the calibration of low level dissolved oxygen monitors. Previously, apparatus for this purpose has usually involved electrolysis techniques with electrodes being placed in the flow of water to produce oxygen by the electrolysis of small amounts of the water. An important disadvantage of present electrolysis methods is that the magnitude of the water flow through the cell must be known precisely before a calculation of the resulting oxygen concentration can be made.
A further disadvantage of certain electrolysis methods is that, due to the high voltages which are necessary in view of the low electrical conductivity of water, a variety of electrochemical reactions can take place of which the electrolysis of water is only one. Consequently there are uncertainties in the concentration of oxygen in the flow. It has been proposed that salts should be added to the water to increase its conductivity, but this is a difficult operation and small uncertainties in the oxygen concentration remain.
It is an object of this invention to provide an improved method and form of apparatus which does not suffer from the above disadvantages and which is simple both in construction of the apparatus and in operation. It will be understood, however, that the invention is applicable to gases other than oxygen and to flows of liquid other than water.
It is a further object of some embodiments of this invention to provide a method and apparatus which affords in a flow of water a concentration of oxygen or other gas of similar physical properties which is substantially independent of not only water flow rate but also water temperature over useful ranges of these parameters.